1. Field of the Invention
The invention relates to a circulating pump unit, consisting of a pump and a motor, for heating and service water installations, in which the motor is settable to at least two different speeds of rotation by means of a rotation speed switch.
2. Description of the Prior Art
As is known, the operating point of a heating or service water installation is determined by the point of intersection between the mains characteristic line and the constriction curve. Circulating pump units to adapt the pump to the systems may be adapted without loss and to save electric drive power have been constructed. With these relatively small motors the variation of the speeds of rotation takes place by series and/or parallel connection of the main and auxiliary windings of the motor. The desired connection is established by means of the rotation speed switch which in general is installed in the terminal box of the pump motor.
Such pumps with selectable rotation speeds have great advantages in view of the fact that they can be driven at low speed in an energy-saving manner during the greatest part of the working time, but they have the serious disadvantage that starting up of the unit is not always guaranteed when the low rotation speed is set.